The somatosensory cortex of adult primates is capable of a surprising degree of functional reorganization following selective sensory deprivation, e.g., after digit amputation (Merzenich et al., 1984) or deafferentation of one upper limb (Pons et al., 1991). After twelve years of deafferentation of one upper limb, for example, the area in 3b corresponding to that hand gets "taken over" by sensory input from the face, i.e., it can be activated by touching the face (Pons et al., 1991). It is remarkable that although somatosensory "plasticity" effects in S1 have been known for over a decade there have been no systematic experiments designed specifically to explore the behavioral consequences of these changes. Our main goal in this proposal is to look for such behavioral effects by studying human patients after amputation of an arm or a single digit. The observation of Pons et al. predicts, for example, that after amputation of one upper limb in a human subject, touching the face should evoke sensations not only in the face but also in specific parts of the missing phantom limb. We will confirm our preliminary observation that this does indeed happen (Ramachandran et al., 1992a,b). Furthermore since the "remapping" observed in monkeys is somatotopically organized we will test the prediction that the points that elicit referred sensations should also exhibit topography. (We will also look for modality specific referral of sensations such as warmth, cold and pain to find out whether remapping can occur separately for each of these modalities.) Remapping increases the cortical magnification; the amount of cortex devoted to a given sensory surface such as the face. We will therefore look for improvements in 2-point thresholds in these areas and try to correlate this with "maps" of referred sensations. To determine whether the changes arise from "sprouting" or from unmasking of preexisting connections we will also study patients immediately after amputation and then follow the time course of these effects. Finally, we will also perform two additional experiments which will critically test our hypothesis that referred sensations arise from the kinds of remapping effects described by Merzenich and Pons. First, we will examine patients after section of the trigeminal nerve (ganglion) to find out of they refer sensations from the hand to the face. Second, we will use MEG recordings to track both the perceptual and the physiological changes over time in the same patient. The demonstration that organized modality-specific "rewiring" can occur in the adult mammalian brain in relatively short periods would have profound implications. Also the experiments may help provide powerful links between physiology and behavior and would have obvious relevance to such clinically important phenomenon as phantom-limb pain.